Magnetic memory devices (magnetic random access memory (MRAM)) including tunneling magnetoresistance effect (TMR) elements as memory elements are now drawing attention as one kind of next-generation nonvolatile memory.
In recent years, practical use of STT (Spin Transfer Torque)-MRAM of a current-driven type has been expected. However, as miniaturization has progressed in the course of increasing capacities, device characteristic variation has become wider, and it is now difficult to maintain the read current, the write current, the transistor current, and a margin between breakdown currents of MTJ elements by reducing variation of each of the currents.
In view of this, an SOT-MRAM utilizing spin orbit interactions (spin orbit torque (SOT)) has been suggested. In the SOT-MRAM, an MTJ element is placed on a nonmagnetic layer (also called an SO layer), and a write current is applied to the nonmagnetic layer, so that the magnetization of the storage layer in the MTJ element is switched. Reading is performed by applying a read current between the nonmagnetic layer and the reference layer of the MTJ element. Accordingly, in the SOT-MRAM, the write current path and the read current path can be separated from each other, and the breakdown current of the MTJ element can be lowered. Thus, the limit on the margin of variation due to miniaturization can be reduced.
However, according to the studies made so far, a nonmagnetic layer having an SOT effect is joined directly to the lower portion of a storage layer. A magnetization switching in the storage layer is caused by the nonmagnetic layer (hereinafter also referred to as the SO layer) placed below the storage layer. Therefore, a nonmagnetic material having a large spin Hall angle is preferably used as the SO layer so as to reduce the reversing current.
Also, as an SOT-MRAM becomes smaller, the influence of field leakage into memory elements of other memory cells becomes larger.
Meanwhile, a structure in which a magnetic layer is interposed between two nonmagnetic layers having spin Hall angles with different signs from each other has also been suggested. There is a report that, with this structure, spin injection into the magnetic layer can be conducted with higher efficiency, and the magnetization of the magnetic layer can be effectively switched. However, there have been neither disclosures nor suggestions of the use of this structure in a magnetic memory.